Relay



Feb. 15,1927. 7. 1,617,377 Y L. N. CRICHTON RELAY Filed July 50. 1920INVENTOR Leslie AZCr/ah 1017 WITNESSES 1 .J X62. MM

' ATTbRNEY Patented Feb. 15, 1927.

' UNITED STATES PATENT OFFICE.

LESLIE N. CBICHTON, F EDGEWOOD PARK, PENNSYLVANIA, ASSIGNOR T0 WESTING-HOUSE ELEC'IRIG & MANUFACTURING VANIA.

Application filed. July 30,

My invention relates to relays and particularly to relays for protectingelectrical distributing systems against grounds.

One object of my invention is to provide a relay system fordisconnecting a feeder circuit when a ground occurs thereon.

Another object of my invention is to provide a relay, for theabove-indicated operation, that shall not be operated by currents .thatare produced by third harmonics.

" A further object of my invention is to provide a power-responsiverelay, for the above operation, that shall be very sensitive to smallvalues of current, and that shall be 1 simple and reliable in itsoperation.

Heretofore, relays for protecting electric circuits against grounds havebeen inserted in the common neutral conductor between currenttransformers and the overload relays. Such relays, however, have notbeen satisfactory because they required too much energy to operate, or,required additional current transformers in systems wherein the ratiosof the transformers that were employed for the instruments and otherrelays were too high for the ground relay. Other relays and systems werenot adequately pre tective because selective o eration was not obtainedwhen the value 0 the ground ourrent was less than the value of normalload current and the relays were not selective because of the diflicultyin obtaining an inverse time element in the operation thereof andbecause the relay had nodirective element. Such relays were furtherundesirable since they were responsive to third harmonies in the systemto which they were connected.

In view of the above conditions, I provide a power-responsive relay thathas its potential winding connected across the terminals of a resistorin the grounded neutral conductor of the system and that has its currentwinding in circuit with the common neutral conductor between the currenttransformers and overload relays or instruments. The power-responsiverelay is thus sensitive COMPANY, A CORPORATION OF PENNSYL- RELAY.

1920. Serial No. 400,093;

to small currents that may result from leakage to ground because of theco-operation between the two windings, and is unaffected by any currentsthat may result from third harmonics in the system.

A ground protecting system embodying this relay overcomes theabove-mentioned disadvantages since little energy is required and sincethe relay is selective, in that it has both a directive and aninverse-timeelement characteristic in its operation, and is notresponsive to third harmonics in the system.

Figure 1 of the accompanying drawings is a diagrammatic view of a systemembodying my invention;

Fig. 2 is a diagrammatic View of the system that is illustrated in Fig.1, illustrating the distribution of the ground current; and

Fig. 3 is a diagrammatic view of a system including a plurality offeeder circuits. A source 1 of electromotive, force supplies energy to asupply circuit 2 from which energy is transmitted to a distributingcircuit 3 through a circuit interrupter 4. The distributing circuit 3 isrovided with a plurality of current trans ormers 5 for energizing apluralit of overload relays '6 and a ground relay The current winding 8of the relay 7 isconnected in circuit with the common neutral conductor9 between the current transformers 5 and the overload relays 6. Thepotential winding 10 of the relay 7 is connected across the terminals ofa limiting resistor 11 that is connected,- by a conductor 12, betweenground 13 and the neutral point of a plurality of potential transformers14 that are connected in Y- relation to the conductors of the supplycircuit 2. i

A safety resistor 15 is provided in parallel relation to thewinding 10of the relay 7 to prevent injury thereto by reason of large potentialdifierences across the resistor 11.

The interrupter 4 is provided with a tripping magnet 16 that isenergized to trip the breaker when therelay 7 operates. The relay 7 isadapted to actuate its contact members 17 to trip the interrupter 4 onlywhen a ground-occurs on a conductor in the distributing circuit 3.

When a ground occurs upon a conductor 18 of the circuit 8 at the pointA, a circuit is completed from the source 1 of electromotive forcethrough the conductor 18, the ground, the ground resistor 11, thepotential transformers 14 and the conductors of the supply circuit 2back to the source 1. Potential is now established across the termi nalsof the ground resistor that serves to energize the potential winding 10of the relay -7. The unbalanced current in the conductor 9, that iscaused by the traversal of current through the ground circuit, encrgizesthe current winding 8 of the relay 7. The two windings of the relay 7co-operate to effect engagement of the contact members 17 and energizethe tripping magnet 16 to trip the interrupter.

A definite distribution of the resulting ground current has beenobserved in such a system, as is illustrated in Fig. 2. The groundcurrent traverses the ground and the grounded neutral conductor 12 andthen divides equally between the three potential transformers. One-thirdof the ground .current traverses each .conductor of the supply circuit 2between the points of connection of the potential transformers 14, andof the conductors of the faulty feeder circuit 3, to the supply circuit2. Beyond the terminals of the feeder circuit 3, onethird of the groundcurrent traverses each of the two normal phase conductors toward thesource 1 of electromotive force. The faulty phase conductor between thefeeder conductor terminal and the source 1 of electromotive force istraversed by twothirds of the ground current. At the terminal of thefaulty feeder conductor, the two-thirds of the ground current combinewith the one-third that traverses the circuit 2 between the potentialtransformer terminal and the faulty feeder conductor terminal.

The effect of the ground current is equivalent to the superposition of asingle-phase electromotive force upon the circuit comprising the faultyconductor as one conductor, and the two normal conductors combined asthe other conductor, of a circuit. One-third (if the ground currenttraverses only the faulty conductor, the ground and the potentialtransformer that is connected to the corresponding conductor in thesupply circuit. This third does not traverse the source 1 ofelectromotive force, but is induced in the primary winding of thetransformer connected to the faulty conductor-by the circulating currentthat has been induced in the secondary windings of the potentialtransformers by the ground currents traversing the primary windings ofthe other two transformers.

The secondary windings of the potential transformers 14 may be employedto provide potential for measuring instruments, etc. and need not,therefore, be of special design or construction.

Since the ground current that traverses the current winding 8 of theground relay 7 is in phase with the current that traverses the groundedneutral conductor 12, the relay 7 may be made so directive and selectiveas to select and isolate a faulty feeder circuit in an electricaldistributing system.

The factors that are required for the operation of the relay 7 are theunbalance effects that are produced both in the currents that traversethe conductors of the feeder circuit 3 and in the effective groundcurrents that traverse the windings of the potential transformers.

Although a generator 1 is shown, it is obvious that the protectivesystem illustrated is readily applicable to a transformer substation andthat modifications may be made in the arrangement of'the current andpotential transformers to obtain the effects required to operate therelay without departing from the spirit and scope of the invention asset forth in the appended claims.

I claim as my invention:

1. In a polyphase distributing system, the combination with a pluralityof current transformers connected in series with the conductors thereof,and a plurality of potential transformers connected between eachconductor and ground through a common current-limiting resistor, of -arelay having two co-operating windings, one of which is responsive tounbalance in the current transformers and the other of which isresponsive to a traversal of current in the currentlimiting resistor.

2. In a polyphase distributing system, the combination with a source ofenergy, a plurality of feeder conductors and an interrupter connectedtherebetween, of a lurality of current transformers energize from thefeeder conductors, a plurality of potential transformers connected fromthe source of energy to ground through a common current-limitingresistor, means for actuating the interrupter and a relay forenergizingthe interrupter actuating means comprising means responsive toan unbalance in the currents traversing the current transformers andmeans responsive to a current traversing the current-limiting resistor.

3. In a pol hase distributing system, the combination with a pluralityof feeder conductors and a plurality of star-connected currenttransformers energized thereby, of

current transformers that is responsive to an unbalance in the currentsthat traverse the current transformer but non-responsive .to thirdharmonics in the system.

4. In a distributing system, the combination with a supply circuit, aplurality of feeder circuits and a plurality of connecting meanstherebetween, of a plurality of watt relays severally responsive to aground current in the feeder circuit and to a corresponding current inthe supply circuit for selectively isolating the faulty feeder circuit.

In testimony whereof, I have hereunto subscribed my name this 19th dayof July,

LESLIE N. CRICHTON.

